Enough Prairie Dogs? Food for Zoo-Raised Ferrets

Ferrets Back on the Prairie

1The black-footed ferret is one of the rarest mammals in North America. By the 1980s it had nearly disappeared from the wild, and the few that were found were brought into captivity to breed. Today, a conservation program run with several zoos raises ferrets and releases them onto grasslands. Before release, young ferrets practice hunting in outdoor pens that have real prairie dog burrows. Seventh grader Theo Brandt spent his summer helping the program's biologist at his city's zoo, Dr. Yolanda Esquivel, sort survey records.

2A ferret is a slim predator with a body about 45 cm long, not counting its tail. It hunts at night, and most of its diet is prairie dogs, which are plant-eating ground squirrels that live in large colonies. Ferrets also sleep and raise their young, called kits, in prairie dog burrows. Biologists estimate that one adult ferret eats roughly 100 or more prairie dogs in a year, and that a mother raising kits needs even more.

3Several years ago, the program released 20 zoo-raised ferrets at each of four sites in the same fall. The sites were similar grasslands, but their prairie dog colonies were very different in size. The sites were far enough apart that ferrets could not travel from one to another. One year later, survey crews counted the ferrets with spotlights at night and weighed the adult females they caught. No disease outbreaks were reported at any of the sites during the study. The results are in Table 1.

4The program also keeps long-term records for Site E, a large colony where ferrets were released earlier. The summer of 2023 brought a drought there. The grass grew poorly, and many young prairie dogs died before fall. Table 2 shows the rain and the prairie dogs counted each summer, and the graph shows the ferrets counted each fall.